Risk Assessment and Simulation Prediction of Groundwater Environment for an Informal Waste Dump in the Karst Region of Southwest China

Karst area has hydrogeological conditions. In this area, the leachate of the informal waste landfills (without anti-seepage layers) will migrate rapidly through the preferential flow paths (karst fissures and conduits) and contaminated the groundwater. Scholars at home and abroad have carried out a series of studies on the characteristics of groundwater pollution from waste dumps or landfills in non-karst areas, and established corresponding pollution assessment, simulation and prediction methods. However, the characteristics, spatial-temporal evolution, risk assessment and prediction of groundwater pollution from informal waste landfills in karst areas with special hydrogeological conditions are still unclear. An informal waste landfill in a karst area of Guangxi was chosen as research object, and a systematically investigation, assessment and prediction of the groundwater environmental pollution were conducted through the way of field investigation, hydrogeological tests, groundwater sampling and analysis, risk assessment and FEFLOW numerical simulation. The results show that the concentrations of permanganate index, chloride, ammonia nitrogen and manganese in the groundwater downstream of the study area exceed the limits of Class IV water quality in the Groundwater Quality Standard (GB/T 14848-2017). Ammonia nitrogen exceeds the standard limit by 17.47 times, and the concentration in the dry season is generally higher than that in the wet season. The horizontal hydraulic conductivity and longitudinal dispersion coefficient of the clay layer in the study area are 1.60 m/d and 0.03 m²/d, respectively. For the limestone layer, the horizontal hydraulic conductivity, longitudinal dispersion coefficient and average flow velocity are 0.255 m/d, 1.4 m²/d and 0.013 m/d, respectively. In addition, multiple fractured zones exist, which facilitate the migration of pollutants into groundwater. The environmental and ecological risk assessment shows that ammonia nitrogen in groundwater of the study area is a heavy pollution indicator, and the downstream groundwater is heavily polluted. The FEFLOW simulation prediction shows that ammonia nitrogen will exceed the downstream boundary of the evaluation scope in about the 12th year, and the pollution plume area will reach 85346 m² in 20 years, posing a potential threat to the groundwater in the downstream area. The numerical model and comprehensive assessment method developed in this study for karst media reveal the spatiotemporal evolution characteristics of groundwater pollution caused by informal waste landfills in karst areas, providing a basis for risk prevention and control of groundwater pollution in such regions.